For the exposure control function DSLRs there are specific shutter speeds available. For example there are 1/4, 1/100, 1/500. Why, even in high end DSLRs, is there no option to use a custom shutter speed (such as 1/19)?

If the camera can do 1/4000, why can't it do 1/33?

Similarly, why are the aperture choices limited?

Why is it a technical challenge to add extra shutter speeds or aperture settings?

  • I read somewhere that the iris in Canon's EF lenses is designed to be set electronically in eighths of a stop (can't find the source sorry) so it's not a technical limitation. – Matt Grum Dec 23 '11 at 22:53
  • @MattGrum, see Wikipedia: F-number – bwDraco Dec 24 '11 at 22:07
  • From the linked Wikipedia article: Modern electronically-controlled interchangeable lenses, such as those from Canon and Sigma for SLR cameras, have f-stops specified internally in 1/8-stop increments, so the cameras' 1/3-stop settings are approximated by the nearest 1/8-stop setting in the lens. This holds for Sigma's lenses for the SA mount because the SA mount used in the Sigma SD15 and SD1 is electronically compatible with the Canon EF mount. – bwDraco Dec 24 '11 at 22:44

Most DSLRs let you choose shutter speed and aperture at 1/3 of a stop difference (3 clicks of the dial to double or halve the amount of light), I'm not a camera designer but I would guess that since 1/3 of a stop is a small difference being able to set exact shutter speed isn't worth the extra electronics and software to support it.

For aperture also add to that the limitations of the camera-lens connection.

So, in other words:

  • 1/19 and 1/33 are nearly identical to 1/20 and 1/30 that are supported.

  • This feature will be slower to operate than just turning the wheel so pro won't use it

  • This feature will be complicated so amateurs won't use it

  • You can use the time it would take to develop this feature to work on features people are actually willing to pay for.

If you were a camera designer what would you have done?

  • To clarify a bit, the main shutter speeds and aperture values are designed to be 1 EV or stop from each other and 1 EV either halves or doubles the light sent to the sensor. So the difference in the light hitting the sensor when going from 1/30 to 1/15 and 1/4000 to 1/2000 is the same -- one stop. – David Rouse Dec 5 '11 at 23:20

Needless complexity.

This isn't a technical challenge, it's a usability one.

Sure the mechanical tooling could be adjusted to do that, and in some high end cell phone cameras you do see some truly bizarre shutter speeds to adapt to their limited range of aperture... but WHY? What technical advantage would it present you to be able to do 1/19th instead of 1/20th? I don't think anyone has made an argument for any case that sufficiently convinces camera makers that it's worth the added complexity.

The same goes for the aperture. If you go back to really really old cameras you'll see continuously variable apertures that could be adjusted to any value. But folks quickly tired of having to do the math to figure out their exposure and started marking spots on the aperture dial to stop the adjustment at that were uniform amounts of light apart. (Here's where we get the concept of an f-stop.) It was just easier to work in evenly spaced units than infinitely adjustable places in between.

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    I guess that 1/19th was a bad example. My camera jumps from 1/2000 to 1/4000. 1/3000 would be pretty nice. I guess my question is why would it be more complex to add more shutter speeds? – SamB Dec 5 '11 at 22:44
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    What camera are you using? Most DSLRs (and later-model electronic-exposure film cameras) will let you adjust in 1/3-stop increments. You usually have to make a custom setting (if it's available) to force full-stop increments. – user2719 Dec 5 '11 at 22:48
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    @SamB - my Canon 550D (an entry level model) goes from 1/2000 to 1/2500 to 1/3200 to 1/4000 - so the missing 1/3000 is specific to your brand or model of camera – Nir Dec 5 '11 at 22:49
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    @rumtscho I didn't know that. I thought it was the difference between 1/4 and 1/8. How is exposure a non-linear relationship (any links)? – SamB Dec 6 '11 at 0:13
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    @rumtscho - your 1/4 to 1/5 example is actually wrong, and surely you meant when SamB wrote. And, just to be precise, it is not the difference but rather the ratio of exposure that is the same in your two examples. – ysap Dec 6 '11 at 1:42

It's convenient for shutter speed, aperture, and ISO to operate in stops, because then it's easy to interchange them. Minus one stop of shutter means plus one stop of aperture or ISO, for example. Using fractional stops adds some versatility, but at the expense of having to cycle through more options. My DSLR lets me choose whether I want operate in third or half stops for that reason.

For auto modes, there's no really compelling reason to require certain speeds anymore. It's probably measured in milliseconds by the firmware anyway. For ISO, the steps are sometimes determined by hardware assumptions, but likewise there's no real reason the chips couldn't br made do different amplifications. However, for both of these, there's usually not a compelling reason to change how it's done. There's just not that much difference in less than a third of a stop of exposure. For aperture, though, some modern lenses in some camera systems are made for video and have smoothly variable aperture, so that exposure and depth of field transitions in a film aren't sudden jumps. (I'm not sure if these lenses can be used in such a manner in still photography mode, though. I wouldn't be surprised if the couldn't, again because the benefit would be small.) In any case, this is often not even possible with the current design of existing lens mounts which tend to communicate aperture in a primitive way.

It's worth noting that some camera companies reserve selection of fractional-stop ISO or shutter speed settings for their higher-end models. There is no technical reason for this. It's just that the full stops are perfectly adequate for almost all situations, and the increased flexibility is used as an incentive to push people into buying the more expensive models.


It's been said a few times before, but I still would like to emphasis the ease of use. Almost everyone who has been interested in photography for a while can tell you the full aperture steps 4-5.6-8-11 etc. Using these values and not the ones between makes it easier for you to know what is happening. It gets harder to remember the values in between. Now let's see what's the difference between 1.2 and 1.4, between 1.4 and 1.8? How many steps are there between 1.4 and 2.4, between 1.4 and 2.8?

I doubt that there are any mechanical or electrical limitations that keep us from having continuous aperture and shutter speeds. It just wouldn't be practical to use. Even the 1/3 or 1/2 steps used today in many DSLRs can be hard to remember.

When taking a picture you sometimes have to make a split second decision and you cannot start doing math then. You must react and not think at a time like that. When the bear comes out from the woods you don't want to think what one stop lower from whatever fraction you are using now is. You just want to take the picture (and maybe run for your life ;-) ).


Traditional shutter speeds and film speeds were based on 10 equal (logarithmically) steps in a scale of 1 to 10. For the range 100 to 1000, these work out to convenient whole numbers: 100 125 160 200 250 320 400 500 640 800 1000. Aperture was traditionally in increments of powers of two: f/2 f/4 f/8 f/16 f/32. These were actually ratios of the square of 2 since an aperture is a two dimensional value (the ratio applies in two dimensions at the same time). So more steps were also added: f/1.4 f/2.8 f/5.6 f/11 f/22. Still ore aperture steps were added to approximate the steps of shutter and film speeds, and that was 3 equal (logarithmically) steps in a scale of 1 to 2 (of the light values between say f/4 and f/5.6).

It's basically an outgrowth of certain convenient numbers for earlier photographers, and adaptations that have been made within those bounds and steps since then. No doubt 10 was involved because of the common number base system (I can't prove it, but I'm sure everyone accepts that).

By comparison, the musical scale is 12 steps in a scale of 1 to 2. This was more about approximating the frequency and harmonic ratios of pleasant tone combinations and steps. And the human ear can discern such a small step of frequency much easier than the human eye could discern 1/12 of a 1 to 2 ratio of light value. Finer aperture or shutter steps might have been needed in some cases for very narrow contrast range film like Tech Pan. Today's film and digital sensors, especially with computer post processing, just don't need any finer exposure steps.

There are sometimes some odd cameras or lenses with "out of step" settings, and often there is a reason. For example there was a lens with a maximum aperture of f/0.95 (yes, less than 1.0 can be done). I doubt very many people could see the difference between f/0.95 and f/1.0 exposures. But at least the specifications were "accurate".


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